PROJECT SUMMARY The olfactory and trigeminal systems are usually considered independent, but most odorants activate the trigeminal system, and most irritants activate olfactory sensory neurons (OSNs). These two systems co-localize in the olfactory epithelium and bulb and in different cortical areas. Trigeminal stimulation usually reduces odor intensity perception, but it has not been established where along the olfactory pathway this inhibition takes place or which physiological mechanisms are involved. The olfactory epithelium, where the olfactory response is first generated, is a candidate locus to harbor this interaction. Here, OSNs are densely interspersed with trigeminal peptidergic fibers. These fibers can be activated by most odorants, and they can release different neuropeptides and ATP. These substances can mediate an inflammatory response, but they can also play other roles. In particular, ATP is an important extracellular signaling molecule, which is involved in neurotransmission in the peripheral and central nervous systems, as well as in the peripheral gustatory system. ATP-sensitive receptors are found in the olfactory epithelium, on both OSNs and trigeminal peptidergic fibers. However, peripheral trigeminal-olfactory interaction and the potential role of ATP as its mediator have not been studied. Using electrophysiological recordings from the entire mouse olfactory epithelium and single mouse olfactory sensory neurons, Aim 1 will clarify how trigeminal activation by an irritant can modify the olfactory response to an odorant and will analyze how this affects responses of single and populations of OSNs. The trigeminal potency of each odorant will be determined, clarifying whether this characteristic has a significant impact on modulation of the olfactory response and its temporal dynamic. Aim 2 will address ATP signaling in the olfactory epithelium, determining trigeminal ATP kinetics in response to odorants with different trigeminal potencies and the role of ATP in modulating the odorant-evoked action potential firing of single OSNs. ATP will be measured using an ATP biosensor. Aim 2 will also determine the role of purinergic currents in olfactory response reduction by irritants, using single OSN recordings from the intact olfactory epithelium. This project will thus provide a more accurate understanding of the role of the trigeminal system as an integral player, together with the olfactory system, in olfactory perception. It will also fill large gaps in our current understanding of a range of poorly understood mechanisms, including perfumery and food preferences and odor and flavor persistence. And it will contribute to understanding the physiology behind pathological variations of olfactory sensitivity, induced by trigeminal activation due to, for example, smoking, hazardous work enviroment or migraine.